The bare conductor of type SE cable is approved for use as a neutral conductor. If used as a neutral it must be connected to the neutral buss. The bare conductor of Type NM cable is not approved for use as a neutral conductor. Using the armor of AC cable or a metal raceway for the neutral conductor has never been permitted.
I have also heard that the reason the neutral conductor was permitted to also serve as the EGC was to conserve metal. Some time ago I had a discussion about this with Bennie Palmer and he said that it was not done to conserve metal.
Curt
